The invention relates, generally, to semiconductor manufacturing and more particularly to attenuated phase shifting reflective lithography.
Currently, transmission optical photolithography is used to form patterned layers in semiconductor manufacturing. Since the ability to resolve the semiconductor devices features during photolithography is directly proportional to the wavelength of the light source, the wavelength of the light source needs to decrease as device dimensions decrease. To pattern device dimensions less than approximately 70 nanometers, one option is to use a light source with a wavelength in the extreme ultra-violet (EUV) regime. As used herein, the EUV regime has a characteristic wavelength between approximately 4-25 nanometers and more specifically 13-14 nanometers. Since it is difficult to find a material that transmits EUV radiation when exposed to wavelengths in the EUV regime, EUV operates in a reflective mode as opposed to the transmission mode. Hence, the EUV masks are reflective in nature and not transmissive like the masks for optical photolithography or other technology options such as Electron Projection lithography or Ion projection lithography.
The concept of attenuated phase shifting has been used in order to improve the resolution of small features in transmission optical photolithography and can be extended to EUV lithography. In transmission optical photolithography, the thickness of the mask substrate is altered to form phase shifting attenuated layers. Changing the thickness of an EUV mask, however, is undesirable because it alters the reflective properties of the mask. One solution to this problem is to form an attenuated phase shifting layer using photoresist as a mask. In practice, however, using photoresist as a mask layer during pattern transfer process generates pattern errors, resulting in the need for inspection and repair of the mask. In this case, repairing the attenuated phase shifting layer can damage the underlying reflective layer as well as alter the phase shifting characteristics, thus decreasing the reflectivity of the mask and consequently making the mask unusable. Thus, a need exists for a process for forming an attenuated phase shifting EUV mask that allows for inspection and repair after photoresist pattern transfer without damaging the reflective layer of the mask.